Network Attached Storage - Advice Please?

The current SBS2003 system I manage uses 2 external hard drives (250
GB) for backing up to. These are swapped daily and one is then taken
off site for protection. These are Seagate FreeAgent Pro USB 2.0/
FireWire 400/eSATA-II drives. I have them connected via an eSATA card
in the server and then a eSATA cable.

I tried USB 2.0 but the backups took ages, so went to eSATA.

The issue I have is that the cable connection to the drive is not 100%
secure. Sometimes the backup fails because it 'cannot find the drive'
so I assume the cable is not connected properly. I have also had to
replace the cable as it appeared to have broke by the connector.

I though about changing to NAS but need to ensure it will work
correctly. I know some of this will be dependent on the drive
purchased.

I am thinking of getting a 1TB drive that will be run in RAID 1.

I want to continue the same process of swapping drives daily.

Will swapping the drives over each day be a simple process?

Will the drive need to be powered down before it can be swapped?

When the drive is swapped will it be given the same allocation letter
so that the back up schedule will not need to be altered?

What else will I need to consider?

Regards

Trevor