I have also met with the same problem recently: some of the fastq file generate "SRR3532923.sra.cache", and I used vdb-validate SRR3532923.sra.cache to check it, which shows:
2020-09-17T20:04:18 vdb-validate.2.10.7 info: Table 'SRR3532923.sra.cache' metadata: md5 ok
2020-09-17T20:04:18 vdb-validate.2.10.7 info: Column 'ALTREAD': checksums ok
2020-09-17T20:04:31 vdb-validate.2.10.7 info: Column 'QUALITY': checksums ok
2020-09-17T20:04:44 vdb-validate.2.10.7 info: Column 'READ': checksums ok
2020-09-17T20:04:45 vdb-validate.2.10.7 info: Column 'X': checksums ok
2020-09-17T20:04:48 vdb-validate.2.10.7 info: Column 'Y': checksums ok
2020-09-17T20:04:48 vdb-validate.2.10.7 info: Table 'SRR3532923.sra.cache' is consistent
Should I trust the download files? Thanks a lot,