I want to convert my 1TB (931GB) and 2TB (1.81TB) external hard drives from FAT32 to NTFS file system to support larger files. I've done some research on how to do it but before I decide if I should it would be really helpful if someone could tell me how much data storage capacity will be lost after conversion?
Not sure about exact numbers for your situation, but the following may help put your mind at ease (from the link above):
NTFS has the following overhead as mentioned by Microsoft:
In general, as your partition size increases, the impact of fixed-size overhead (FAT tables, NTFS metadata) becomes less significant compared to the impact of cluster-overhead (because FAT's clusters can get very large on large partition sizes, but NTFS's don't get as large.)
For something very large, like 1TB, my gut feeling is that the NTFS format will let you store more data, not less.
I have never taken the time to run the analysis for FAT32 vs. NTFS, but back in 1995, I analyzed FAT16 vs. OS/2 HPFS to come to this conclusion. My USENET post with the result may be found here: https://groups.google.com/d/msg/comp.os.os2.misc/zZ84hesFXRE/Kf1c-1qlXcEJ.
Given that FAT32's ancestor is FAT16 (larger FAT tables, but shares the same basic architecture) and NTFS evolved from HPFS, I would expect the conclusion to be the same, even though the details will certainly be different.