It seems that faulty RAM or even a faulty cable could adversely affect either creation or verification of backups.
May be worth testing your RAM -
best-free-memory-testing-utility
Also try a different cable maybe?
How is your external drive formatted - NTFS or FAT32?
With FAT32, if the images are larger than 4GB then they would presumably be split up which theoretically might increase the chance for errors to occur?
Additionally, if you choose the highest compression settings this again could potentially increase the chance for errors?