allow drive letters A and B as mount targets

src/main/java/org/cryptomator/common/mountpoint/AvailableDriveLetterChooser.java

@@ -24,6 +24,6 @@ class AvailableDriveLetterChooser implements MountPointChooser {
 
 	@Override
 	public Optional<Path> chooseMountPoint(Volume caller) {
-		return this.windowsDriveLetters.getAvailableDriveLetterPath();
+		return this.windowsDriveLetters.getDesiredAvailableDriveLetterPath();
 	}
 }

src/main/java/org/cryptomator/common/vaults/WebDavVolume.java

@@ -70,7 +70,7 @@ public class WebDavVolume implements Volume {
 		//on windows, prevent an automatic drive letter selection in the upstream library. Either we choose already a specific one or there is no free.
 		Supplier<String> driveLetterSupplier;
 		if (System.getProperty("os.name").toLowerCase().contains("windows") && vaultSettings.winDriveLetter().isEmpty().get()) {
-			driveLetterSupplier = () -> windowsDriveLetters.getAvailableDriveLetter().orElse(null);
+			driveLetterSupplier = () -> windowsDriveLetters.getDesiredAvailableDriveLetter().orElse(null);
 		} else {
 			driveLetterSupplier = () -> vaultSettings.winDriveLetter().get();
 		}

src/main/java/org/cryptomator/common/vaults/WindowsDriveLetters.java

@@ -22,11 +22,11 @@ import java.util.stream.StreamSupport;
 @Singleton
 public final class WindowsDriveLetters {
 
-	private static final Set<String> C_TO_Z;
+	private static final Set<String> A_TO_Z;
 
 	static {
-		try (IntStream stream = IntStream.rangeClosed('C', 'Z')) {
-			C_TO_Z = stream.mapToObj(i -> String.valueOf((char) i)).collect(ImmutableSet.toImmutableSet());
+		try (IntStream stream = IntStream.rangeClosed('A', 'Z')) {
+			A_TO_Z = stream.mapToObj(i -> String.valueOf((char) i)).collect(ImmutableSet.toImmutableSet());
 		}
 	}
 
@@ -35,7 +35,7 @@ public final class WindowsDriveLetters {
 	}
 
 	public Set<String> getAllDriveLetters() {
-		return C_TO_Z;
+		return A_TO_Z;
 	}
 
 	public Set<String> getOccupiedDriveLetters() {
@@ -59,6 +59,26 @@ public final class WindowsDriveLetters {
 		return getAvailableDriveLetter().map(this::toPath);
 	}
 
+	/**
+	 * Skips A and B and only returns them if all other are occupied.
+	 *
+	 * @return an Optional containing either the letter of a free drive letter or empty, if none is available
+	 */
+	public Optional<String> getDesiredAvailableDriveLetter() {
+		var availableDriveLetters = getAvailableDriveLetters();
+		var optString = availableDriveLetters.stream().filter(s -> !(s.equals("A") || s.equals("B"))).findFirst();
+		return optString.or(() -> availableDriveLetters.stream().findFirst());
+	}
+
+	/**
+	 * Skips A and B and only returns them if all other are occupied.
+	 *
+	 * @return an Optional containing either the path to a free drive letter or empty, if none is available
+	 */
+	public Optional<Path> getDesiredAvailableDriveLetterPath() {
+		return getDesiredAvailableDriveLetter().map(this::toPath);
+	}
+
 	public Path toPath(String driveLetter) {
 		return Path.of(driveLetter + ":\\");
 	}