differential backup backups all changes since last full backup,
therefore in your scenario you don't lose any data.

If you would restore the database, the procedure is:

1- backup the tail of the log (if it's not corrupted)
2- rstore full
3- restore last diff
4- restore backuped log (from 1)

please read more in BOL

it wouldn't hurt to move/rename old diff when doing new one - instead of
overwrite. Consider the case when diff backup doesn't finish OK. then
you don't have any diff backup 'cause:
1- old file was overwritten
2- new one wasn't finished

Scott
BOL says
"A differential database backup records only the data that has changed since
the last database backup. You can make more frequent backups because
differential database backups are smaller and faster than database backups.
Making frequent backups decreases your risk of losing data."

Test  your DDR

create database test
GO
create table test..test(id int identity)
insert test..test default values

backup database test to disk = 'c:\db.bak' WITH INIT
insert test..test default values
backup database  test to disk = 'c:\db_diff1.bak' WITH DIFFERENTIAL
insert test..test default values
backup database  test to disk = 'c:\db_diff2.bak' WITH DIFFERENTIAL

GO
RESTORE DATABASE test FROM disk = 'C:\db.bak' WITH FILE = 1, norecovery
---RESTORE DATABASE test FROM disk = 'c:\db_diff1.bak' WITH FILE = 1,
recovery
RESTORE DATABASE test FROM disk = 'c:\db_diff2.bak' WITH FILE = 1, recovery

select * from test..test

DROP Database test

Hi,

A DIFF backup contains all changes since the last FULL backup, so you
won't loose anything by skipping a DIFF backup file. Actually, in a
restore scenario, you will restore the latest FULL backup and the latest
DIFF backup - you don't need the DIFF backups in between.

Personally I prefer to backup up to seperate files rather than
overwriting existing files. By backing up to seperate files, I'll always
have the older files available if one of the files suddenly becomes
corrupt. If you keep overwriting and only have one file, then you've
lost everything if this file is damaged.
I know there might be cases where you can get older files from a
filebackup, but most companies only run file backups nightly. This means
that if the file is being corrupted in evening, then you've lost the
whole days backup.

You might also want to read up on BACKUP/RESTORE in BOL - that will
explain the different options you have.

Regards
Steen Schlüter Persson
CRM System Specialist / DBA