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String or binary data would be truncated. The statement has been terminated(EF 5.0)

We have a small old app which transfers data from one Server to another server (Using Entity Framework 5.0 and SQl server ) and generate reports issue I am having is that when it is transferring data. I am getting ("InnerException = {"String or binary data would be truncated.\r\nThe the statement has been terminated."}"..
on some data...but if I want to enter that particular data with "id" using the same app it is inserting it with no issue.I read somewhere it is an issue with EF(5.0) but I am not sure so Please let me know.

You have a char / varchar / nchar / nvarchar column with a specified length. You are inserting a row where the value has a length greater than the specified length.
When you read it from the old server, you should use string.Substring to get the first X characters from the source (where X is the database column length). If you do this, your errors will stop.

Store huge amount of data in SQL Server

create table article
(
ArticleID int constraint cnst-name Primary key,
Description datatype
)
I am creating this table in SQL Server 2014. I am trying to create a table where I can store articles with huge data (it can be 1000-2000 words article description). I don't know which data type to choose for description column.
I chose varchar(max) but there is a limitation that each row has to be <= 900 bytes. Please guide me if my table structure is right.
Thanking in anticipation.

Use VARCHAR(max).
2000 words in a .txt file is around 13kb.
I would not call that 'huge'.

I would use nvarchar(max). Unlike varchar is supports unicode. The limit 2GB should be enough.
The limit of row size is 8060B, so You can not store more than about 4000 unicode characters in the row. But the restriction does not apply here, because nvarchar(max) is not stored in the row. The row contains pointer only. This one indirection is the price for the "unlimited" size.

Not able to save more than 43679 char in text datatype column in SQL Server

I have a column in table with text datatype and trying to save some string value to this column from C# code. Issue comes when I use some very large string.
I am not able to save more than 43679 character into text field. I know text size can be 2^31.
I also tried saving same value from SSMS and noticed same scenario.
There is nothing special about code, but still SQL query is given below...
update TableName
set ColumnName = 'some text more than 43679 char'
where id=<some int id>
just to mention... column is declare in table as
[columnname] [text] NULL
Can anyone tell me what could be wrong.

You can try to use varchar(max) to store huge amount of data. See MSDN
We recommend that you store large data by using the varchar(max),
nvarchar(max), or varbinary(max) data types. To control in-row and
out-of-row behavior of these data types, use the large value types out
of row option.
You can also check the same issue here: SSMS - Can not paste more than 43679 characters from a column in Grid Mode

Get Length of Bytes stored in a data column which stores binary data

I have a data column in my table that stores "image"(SQLDBType) type i.e . it has binary data.
I want to read the value of the above column in SQLDataReader through C#.
I used the function
dr.GetBytes(6,0,Data,0,length);
6 -> my column no.
0 -> start index from where to read
data -> buffer where i want to read
0-> index in buffer where i want to copy
length -> length of the data to be read.
now i want to get the length of the binary data in the datacolumn (here, i have used length variable above).
How to do it?

You can add another column to your query that uses the SQL DATALENGTH function. Read that column's value first, then allocate your buffer and use GetBytes.
By the way, you should be migrating away from image.
ntext , text, and image data types will be removed in a future version of Microsoft SQL Server. Avoid using these data types in new development work, and plan to modify applications that currently use them. Use nvarchar(max), varchar(max), and varbinary(max) instead.

You could let the database tell you that - eg
SELECT data, DATALENGTH(data) AS DataSize
FROM tableOfData
WHERE id = yourId

Convert varchar to nvarchar

My company use sql server 2000 to store data . There is a table with a column named 'Vattu' .
The problem is that : this column declare as varchar data type , however it's save both unicode and anscii value !
So every time I show this column data on web , it show unreadable characters.
Is there any way to convert data to unicode value using c# ?

A quick google search would have given you the answer. Anyway you use System.Text.Encoding to encode the ascii to unicode. check the sample code at Encoding.Convert Method
Also converting the column datatype to NVarchar will be better in long run. Doing so you will save CPU processing that you will be using due to conversion at c# level.

You need to change the data type of the column to nvarchar in order to store Unicode data.
The Unicode data you've already tried to store in the column is gone - you can't magically get it back, unless your application logged the data or you were running traces of all commands and still have access to those.