USB2.0 in practice for external devices maxes out at almost exactly 32MBps.
USB3.0 in practice is bottlenecked solely by the HDD itself and what data is being accessed (smaller pictures files could be closer to 2MBps). Max speeds for larger, sustained transfers could be over 130MBps.
Other:
I've had a lot of issues with SEAGATE, and investigated and IMO (I could be wrong) Western Digital has better USB drives.
*So choose a drive that:
1) uses USB3.0
2) is 7200RPM (if you know)
3) capacity fits your needs/budget (2TB?)
4) Size fits your needs/budget
2.5" is slower but more portable. A 7200RPM 2.5" drive is slower than a 3.5", and the average speed of a LARGER capacity drive is also higher... that's due to the rotational speed of the OUTER EDGE of the drive platters...
The data is more likely to be on the outer edge if you have more capacity, and 3.5" has a larger diameter so it travels faster (at same RPM).
OTHER:
WD also makes a great WDMYCLOUD device if you want it just for the home. It attaches to the ROUTER via Ethernet cable and becomes accessible to all wi-fi or ethernet media/PC devices... I use it to stream MOVIES.
It's fast (up to 90MBps).
SUMMARY:
Your choices should be:
a) USB3.0 2.5" (portable)
b) USB3.0 3.5" (semi-portable)
c) WDMYCLOUD (local; all devices)
3.5" HDD's tend to have AC adapters as well, whereas 2.5" HDD's do not.