The most compatible HD / SSD file format is FAT32, this is the common format for USB sticks. But: there are size limitations: partition max = 2TB and 4 Gb per file max. Better format though is exFAT32, there is no file size limitation and it works on  all platforms like PC, Linux and Mac. As we work here on Macs, you can stick to Mac OS X standard format HFS+ if you are MAc user to. Pls do not bring your HD / SSD in Windows NTFS format!