Apple Partitioned External Hard Drive Issue

I am using an external hard drive that has been apple HFS partitioned.

When I connect it to the Imac & goto to transfer files from the Imac to the external hard drive it takes forever ?

Ive correctly disconnected the external HD then Ive gone to system preferences - clicked on the start up disk option & highlighted the option to restart using the MAC operating system only.

(No external drives show in this option anyway)

After restart & then reconnecting the ext HD the issue is still there ?

The external drive is not connected to the mac on boot up as this seems to confuse the Imac as to which is the slave & master.

A final check was to use another ext HD connected to the Imac - partitioned in HFS & this works fine !

Any ideas ??

The drive in question was working a while back but has reverted back to slow mode ??

You do not give the Imac specifications, and it could be a G3, G4, G5 or Intel Dual Core. For PowerPC machines, G3, G4 and G5, Firewire is the go as USB, even USB2 is too slow, and the machine will only boot from the external HD if firewire is used.

Set up the external drive by using Apple Partition Map, selecting one or more partitions, and then format using Mac OS Extended (Journaled).