Currently I am using apex data loader with bulk api enabled and batch size =1 . But its giving me failed to create batch when i try to import 1000 files with each size around 100kb. I need to migrate around 100,000 attachments. Can't do it manually or in batch of 10 or 100. What's the best way to do it ?
If Possible don't use bulk API. Make Bulk Api disabled while using apex data loader. Bulk Api has lot of governor limits. And Try Also keeping batch size as low as 1 if upload attachment size is big.