Question : Problem: Portable Storage and durability

I'm trying to figure out the best exernal HD for our needs. We want to add external HDs for offsite backups (still using a tape for some critical data). My thinking was that we would get 2 external HDs that would be exchanged each day (we currently do this with tapes, rotating daily so that every night one tape is offsite with the previous day's backup). The other advantage of adding HDs to our backup system is more storage space.
Is there any significant difference in durability between standard external HDs (with 3.5" disks inside), and 'portable' external HDs with 2.5" disks inside? My understanding is that the larger format external HDs are intended for primarily stationary use, so I'm not sure how they would hold up with being carried back and forth every day. On the other hand, the 2.5" 'portable' drives are often billed as ideal for additional storage to carry around with a laptop.
My boss is the one who usually brings the tape back and forth to the office every day. He doesn't plan on using the external HDs for traveling or carrying around all the time, so he's not too concerned about the larger size and weight.
Any insights or suggestions would be appreciated.

Answer : Problem: Portable Storage and durability

the smaller enclosures with 2.5 disks that have the laptop drives supposedly are supposed to be designed more for that.
take a look at the durability of the IPOD it seems to work very well with traveling

tapes are less likely to fail and are easier to recover data from if they get damaged and they dont suffer much if you drop one. it does have a lot of moving components like a harddrive.

If for instance there was a major earthquake, a tape would have significate shock resistance.
A harddrive may suffer.

you may be able to buy harddrive enclosures that are impact resistant or you can make them homemade
but just have an alternate backup as well or an offsite backup like leew suggested