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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?

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This KB article may give you some insight: – nixda Jul 17 '13 at 18:16

Not sure about exact numbers for your situation, but the following may help put your mind at ease (from the link above):

Like FAT, the NTFS file system has a certain amount of fixed-size overhead and a certain amount of per-file overhead. NTFS also has several advanced features, including recoverability, security, and support for very large volumes. Because of these advanced features, the NTFS metadata overhead is somewhat larger than the FAT metadata overhead. On the other hand, because NTFS cluster overhead is smaller than FAT cluster overhead, it is often possible to store at least as much on an NTFS volume as on a FAT volume, even without using NTFS file compression.

NTFS has the following overhead as mentioned by Microsoft:

It is not recommended to use NTFS on a volume that is smaller than approximately 400 MB, because of the amount of space overhead involved in NTFS. This space overhead is in the form of NTFS system files that typically use at least 4 MB of drive space on a 100 MB partition.

For Fat32, it depends on the block size and the parition size:

Blocksize     Overhead on 2GB parition (in MB)
512           25**
1024          24**
2048          35**
8192          119
32768         470

(**) You aren't allowed to use these blocksizes for a 2 GB 
     partition although it probably would be possible.

The table gives you an estimate on the amount of overhead in MB for each of the filesystems described above, when they need to store 30000 files varying in size from 4 to 1000 kB (there are more small files than large files). This includes the overhead for the bitmap or FAT, the overhead for administration blocks and the overhead caused by having to round the size of files up to the nearest multiple of the blocksize. The size of the partition used is almost 2 GB.

Keep in mind that these are calculated figures, and that they represent an average case ... Also keep in mind that a lot of things have been simplified, but these figures should still be accurate to within a couple of megabytes.

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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:

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.

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