Windows 7: external hard drives hang in W7 64 bits

Hi all,
I purchased Windows 7 ultimate and made a dual boot Windows 7 32 bits and Windows 7 64 bits. I was hoping to eliminate the 32 bit version once the 64 bit version was ok. For back ups, I use 2 external drives: a Mybook Studio 1 TB and an Iomega 750 MB. The two drives work perfectly well under Windows 7 32 bits, but under the 64 version, Windows 7 hangs after a while, when trying to copy files to the drive. Reading from the drive is not a problem though. When activating the Mybook drive, it shows up correctly but instead of the usual choice of actions, offered by windows, a message says "H:\ application not found". Reading from this drive is not a problem, but writing to it also makes Windows 7 hang. I tried installing a windows hotfix for 64 USB problems, I tried installing new drivers for the Mybook to no avail. Help would be much appreciated.
Kind regards,
Jos