TRS-80 single density 5.25" disk
The TRS-80 single density 5.25" disk was the main disk format officially used with TRS-80 Model I/III/4 computers. (A similar format was also used on the Model II, but this model was incompatible with the others.) When the Model I was first released in 1977, cassettes were the storage medium used, but a year later the disk drive was released. It was a single sided, single density drive. Later, third-party add-ons provided support for the TRS-80 double density 5.25" disk, which eventually became an official upgrade.
The TRSDOS operating system was used.
Normal single density disks had 35 tracks, but later third-party drives had 40 tracks for an increased storage capacity. Each track has 26 sectors with 128 bytes per sector (or maybe 256? Some references have that). The TRSDOS file system (in various versions) was used.
Single density disks can be difficult to read for archival purposes in the present, since so few disk drives and controllers are around that support that obsolete density; most later 5 1/4" drives are designed to support only the double and high density formats.